of Edinburgh, Session 1880 - 81 . 
31 
corpuscles have been observed (Stricke) ; and in cell division, which 
is always associated with very slow change of shape or movement, 
the stroma certainly alters in a very marked manner (Flemming, 
Klein). 
It may at first sight be thought that a pseudopodium is produced 
by a change (either of shortening or elongation) of part or the 
whole of the stroma, whereby a part of it protrudes as a 
process. This may be the case, but there are certain difficul- 
ties in accepting this view. The stroma is a closed network, and 
therefore the meshes must be broken up in order to allow of a 
portion of it at the surface to be separated from the rest in the form 
of a process ; the superficial parts must be, in fact, torn away from 
those subjacent. Besides this, the substance of the pseudopodium 
is more fluid than that of the cell mass, looking in general very 
hyaline and uniform, although in some cases granules may be 
observed in its substance. The pseudopodia, moreover, are often 
very fine, quite as fine in fact as a strand of the meshwork itself. 
Although no doubt a cell can change very considerably in form, 
due to the contractility of the stroma, the relations between the 
stroma and interstromal matter being inconsiderably altered, yet 
probably many at least of the pseudopodia, for the reasons given 
above, are formed in another way. They consist probably of the 
interstromal matter, or portions of it, projected outwards by the 
contractions of the stroma, which I imagine to occur in the follow- 
ing manner : — The stroma contracts at every part except where the 
pseudopodium springs from, forcing the interstromal matter at this 
point through the aperture left patent. 
This accords well with the fact that the pseudopodia seem 
actually to be projected always as radia from the cell, and that they 
are of a very hyaline nature. The difficulty is to comprehend the 
forces engaged in their retraction. There are probably at least three, 
— (1) the relaxation of the stroma; (2) the viscosity of the sub- 
stance, and (3) surface tension, in virtue of which a body tends to 
assume the spherical shape. 
Now this may be very well theoretically, but are these three 
factors equal to the occasion is the question before us ? I have 
imitated the structure of the amceba in the following way : — 
